Vermetid snails are bad for stony corals, coral crabs keep em in check
We always believed that vermetid snails were bad for corals in aquariums, but now there is real scientific evidence and observation to support the same conclusion about corals in the wild. In case you are not yet familiar with them, vermetid snails are a type of gasdtropod which often lives in close association with corals and other reef structures, and they are a type of snail that grows its shell on the reef or on corals, rather than growing a discrete shell and growing around. Also, instead of going around grazing the reef as most snails do, vermetid snails release and retrieve a net of mucus which traps food, but it also rubs up on corals and that’s where we start disliking them.
Vermetid snails are so common on the reef and in aquariums, it’s a surprise that we rarely have any kind of open discussion about them. However, the effects of Vermetids on reef corals were not lost on coral scientists who recently quantified the net effect of Vermetid snails on different coral species, and the results aren’t good. Where vermetid snails were found, coral cover was dramatically decreased and more importantly, rugose and more gnarly-shaped corals developed more flattened growth form, leading to more sedimentation, less particle capture and less polyps overall to be able to grow and reproduce on behalf of the coral colony. More specifically, Jeffrey Shima and his colleagues found that in Moorea, “Vermetids reduced skeletal growth of focal corals by up to 81 percent and survival by up to 52 percent.”
Results like that are enough to ban all vermetids from our dozen home aquariums and if we didn’t tolerate them before, now vermetids will be on a zero tolerance policy. Wild coral colonies are not defenseless however and with a nice armada of coral crabs, the effect of vermetid snails can be kept in check. Coral crabs have not been observed to interact with vermetid snails in any way, but their ‘housekeeping’ activities on the coral colony helped to remove or dislodge the mucus net which would otherwise stress and smother coral branches. As you can see in the video below, vermetids have a fascinating mode of feeding but it doesn’t jive with corals and therefore we don’t like it either.
Jeffrey S. Shima, Craig W. Osenberg and Adrian C. Stier. 2010. The vermetid gastropod Dendropoma maximum reduces coral growth and survival. Biology Letters. Volume 6, pp. 815-818.